Durability of GFRP and CFRP Bars in the Pore Solution of Calcium Sulfoaluminate Cement Concrete Made with Fresh or Seawater

Calcium sulfoaluminate cement concrete (CSAC) reinforced by fiber-reinforced polymer (FRP) bars, termed bars for brevity, is a good alternative to steel-reinforced concrete in marine environments due to the corrosion resistance of FRP and the lower pH of CSAC. For the first time, multi-mechanical tests are conducted to compare the durability of glass FRP (GFRP) to that of carbon FRP (CFRP) after exposure to CSAC pore solution. The bars were immersed in a simulated pore solution of CSAC made with either fresh water and river sand or with seawater and sea sand. Solution temperature was held constant at 30 °C, 45 °C or 60 °C for 30, 60, 90 and 180 days of immersion. Tensile, horizontal and transverse shear tests, as well as detailed microstructural analyses, were conducted to determine the level and mechanisms of degradation for each type of bar. Sea salt increases the degradation of both bars, but it degrades GFRP more than CFRP. The bars’ retained tensile strength is a reliable indicator of their durability, while their post-exposure horizontal and transverse shear strengths are found inconsistent and counter intuitive. In the GFRP, the fiber, the epoxy matrix and their interface suffered damage, but in the CFRP, the carbon fiber was not damaged. Under the test conditions in this study, the maximum reduction in the tensile strength of the GFRP was 56.9% while that of CFRP was 15.1%. Based on the relevant ASTM standard, the CFRP bar satisfies the alkaline resistance requirement of the standard in the CSAC pore solution with and without salt, whereas the GFRP bar does not meet the same requirement in the above pore solution with salt.


Introduction
Fiber-reinforced polymer (FRP) bars are characterized by low density, high strength and good corrosion resistance. They are increasingly used as replacement of steel bars in concrete structures exposed to corrosion inducing agents. The durability of FRP bars exposed to sea/tap/deionized water and the alkaline environment of Portland cement (PC) concrete (PCC) has been extensively and systematically investigated over the past two decades [1][2][3][4][5][6][7][8][9][10]. One of the characteristics of PCC is its high pH value, but experiments have shown that FRP, especially glass FRP (GFRP) and basalt FRP (BFRP), are more resistant to degradation in a low-pH environment. The latter is demonstrated by their relatively higher retained horizontal shear [4,5], tensile [3,[6][7][8] and flexural [8] strengths.
Methods to reduce the pH of concrete include addition of pozzolanic materials [3] to PC or replacement of PC by calcium sulfoaluminate cement (CSA) [7] due to the normally lower pH of CSA concrete (CSAC) [7,[11][12][13][14][15][16]. CSA has been the subject of large-scale As shown in Figure 1, GFRP and CFRP bars composed of E-glass/carbon fibers and epoxy resin were acquired from the same manufacturer in China. The bars were manufactured using the pultrusion method [53]. The detailed physical and mechanical properties of unexposed or reference GFRP(GR) and CFRP(CR) bars, determined in the current study based on the relevant ASTM standards, are shown in Table 1. Note, each row of column 3 of the table refers to the relevant ASTM standard for determining the companion physical or mechanical property, which is specified in column 2 of the same table.
on the durability of these bars.

Materials
As shown in Figure 1, GFRP and CFRP bars com epoxy resin were acquired from the same manufactu factured using the pultrusion method [53]. The detaile ties of unexposed or reference GFRP(GR) and CFRP(C study based on the relevant ASTM standards, are sh column 3 of the table refers to the relevant ASTM stand physical or mechanical property, which is specified in

Immersion Solution Chemistry
The GFRP and CFRP bars were immersed in simulated CSAC pore solutions, termed PS and SS. Based on data in [11,12], PS had the chemical composition of the pore solution of a CSAC made with fresh water and cleaned sand, while SS had that of a CSAC made with seawater and sea sand. The PS principal chemical components, as shown in Table 2, were obtained in [11] by analyzing the pore solution of CSAC. The simulated seawater chemical composition in Table 2 is specified by ASTM D1141-98 [57]. As can be noticed in Table 2, the SS contains the same chemicals as PS plus the chemicals in simulated sea water. The two solutions have the same pH value. All chemical materials, except potassium hydroxide, were analytical pure, while the purity of potassium hydroxide was greater than 85%.

Test Setup
The setup for conditioning the FRP bars, henceforth referred to as bars for brevity, in the solutions is illustrated in Figure 2a,b. In total, six identical setups were used. A typical setup involved a rectangular plastic tank filled with one of the solutions. Holes were drilled in the two opposite walls of the tank to pass the 1000 mm long bar samples through the tank, and then the holes were sealed with silicon and water-resistant tape. The width of the tank was 200 mm, which is equal to the length of the conditioned part of each bar sample. Thus, as shown in Figure 2, approximately 400 mm long segments of the bar projected from two walls of the tank. An L-shaped stainless-steel pipe, Figure 2b, was placed inside the tank and a heating element was inserted into the pipe. The pipe was filled with water that was heated by the heating element. The solution temperature was maintained constant via a thermostat with a precision of ±1 • C, and the water level in the pipe was controlled via an automatic controller.

Test Setup
The setup for conditioning the FRP bars, henceforth referred to as bars for brevity, in the solutions is illustrated in Figure 2a,b. In total, six identical setups were used. A typical setup involved a rectangular plastic tank filled with one of the solutions. Holes were drilled in the two opposite walls of the tank to pass the 1000 mm long bar samples through the tank, and then the holes were sealed with silicon and water-resistant tape. The width of the tank was 200 mm, which is equal to the length of the conditioned part of each bar sample. Thus, as shown in Figure 2, approximately 400 mm long segments of the bar projected from two walls of the tank. An L-shaped stainless-steel pipe, Figure 2b, was placed inside the tank and a heating element was inserted into the pipe. The pipe was filled with water that was heated by the heating element. The solution temperature was maintained constant via a thermostat with a precision of ±1 °C, and the water level in the pipe was controlled via an automatic controller. In the current investigation, the bars were conditioned under constant temperatures of 30, 45 and 60 °C for periods of 30, 60, 90 and 180 days. The immersion durations and the highest immersion temperature in this study are based on the ASTM D7705/D7705M-12 [48] recommendation. It states that specimens for procedure A, the one applied in this study, shall be immersed in the alkaline solution at 60 ± 3 °C (140 ± 5 °F) for exposure times of 1, 2,3 or 6 months, unless longer exposure periods are specified. The temperature of 30 °C in this study is selected mainly based on the annual average surface temperature of seawater in the South China Sea [58]. Since the functional relationship between the longterm durability of FRP bars as a dependent variable, and the immersion temperature and duration as independent variables requires a minimum of three values [59] for each independent variable, the 45 °C temperature, which lies midway between 30 and 60 °C, was chosen as the third immersion temperature. It is important to note that three similar In the current investigation, the bars were conditioned under constant temperatures of 30, 45 and 60 • C for periods of 30, 60, 90 and 180 days. The immersion durations and the highest immersion temperature in this study are based on the ASTM D7705/D7705M-12 [48] recommendation. It states that specimens for procedure A, the one applied in this study, shall be immersed in the alkaline solution at 60 ± 3 • C (140 ± 5 • F) for exposure times of 1, 2,3 or 6 months, unless longer exposure periods are specified. The temperature of 30 • C in this study is selected mainly based on the annual average surface temperature of seawater in the South China Sea [58]. Since the functional relationship between the long-term durability of FRP bars as a dependent variable, and the immersion temperature and duration as independent variables requires a minimum of three values [59] for each independent variable, the 45 • C temperature, which lies midway between 30 and 60 • C, was chosen as the third immersion temperature. It is important to note that three similar temperatures were also used in [6,[59][60][61][62] to investigate the durability of FRP bars in conventional concrete pore solution.
Companion virgin GFRP and CFRP bars were tested as references and were designated as GR and CR, respectively. For easy reference, the conditioned bars are designated as BTSTT#D#, where BT = bar type = C for carbon and G for glass, ST = solution type = PS or SS, T# = conditioned solution temperature in • C, e.g., T30, D# = the length of the conditioning period in days, e.g., D60. For example, CSST60D90 represents the group of CFRP bars immersed in the SS solution under constant 60 • C for 90 days.

Mechanical Tests
The rigs for the tensile, horizontal and transverse shear tests of the bars are shown in Figure 3a-c, respectively. To determine the tensile and horizontal shear strengths, six replicate specimens were tested in each case, while for transverse shear, five specimens were tested. temperatures were also used in [6,[59][60][61][62] to investigate the durability of FRP bars in conventional concrete pore solution. Companion virgin GFRP and CFRP bars were tested as references and were designated as GR and CR, respectively. For easy reference, the conditioned bars are designated as BTSTT#D#, where BT = bar type = C for carbon and G for glass, ST = solution type = PS or SS, T# = conditioned solution temperature in °C, e.g., T30, D# = the length of the conditioning period in days, e.g., D60. For example, CSST60D90 represents the group of CFRP bars immersed in the SS solution under constant 60 °C for 90 days.

Mechanical Tests
The rigs for the tensile, horizontal and transverse shear tests of the bars are shown in Figure 3a-c, respectively. To determine the tensile and horizontal shear strengths, six replicate specimens were tested in each case, while for transverse shear, five specimens were tested.

Tensile Test
The tensile test was conducted in accordance with ASTM D7205/D7205M-21 [51] specifications using a 100 kN universal testing machine. Steel tubes that were 300 mm long were used to anchor the bar ends, and the tubes were grouted with a mixture of silica sand and epoxy. The bar free length between the anchors was 400 mm, with the middle 200 mm being the conditioned part. The bar extension was measured with an extensometer with gauge length of 50 mm.

Horizontal Shear Test
The horizontal shear test was conducted based on ASTM D4475-21 [49] using the rig in Figure 3b, which complies with the requirements of ASTM D4475-21 [49]. The span-todiameter ratio was 3 for conditioned bars; the basis for the selected ratio will be explained later. Before the test, the diameter of each specimen was measured at its midspan using digital calipers with 0.02 mm accuracy. Using the same universal testing machine that was

Tensile Test
The tensile test was conducted in accordance with ASTM D7205/D7205M-21 [51] specifications using a 100 kN universal testing machine. Steel tubes that were 300 mm long were used to anchor the bar ends, and the tubes were grouted with a mixture of silica sand and epoxy. The bar free length between the anchors was 400 mm, with the middle 200 mm being the conditioned part. The bar extension was measured with an extensometer with gauge length of 50 mm.

Horizontal Shear Test
The horizontal shear test was conducted based on ASTM D4475-21 [49] using the rig in Figure 3b, which complies with the requirements of ASTM D4475-21 [49]. The span-todiameter ratio was 3 for conditioned bars; the basis for the selected ratio will be explained later. Before the test, the diameter of each specimen was measured at its midspan using digital calipers with 0.02 mm accuracy. Using the same universal testing machine that was where S is the horizontal shear strength in Pascal (N/m 2 ), P is the applied breaking load in N, and d is the bar diameter in m.

Transverse Shear Test
The transverse shear test was conducted in compliance with ASTM D7617/D7617M-11 [50] specifications using the rig in Figure 3c. The cross-sectional area was measured as per ASTM D7205/D7205M-21 [51]. Specimens were loaded at a rate of 1.0 mm/min based on the machine crosshead movement. The transverse shear strength was calculated per ASTM D7617/D7617M-11 [50] using Equation (2).
where τ U is the transverse shear strength in MPa, P S is the maximum or failure force in N, and A is the bar cross-sectional area in mm 2 .

Microscopic Analyses
To obtain the bars deterioration evolution and mechanisms after exposure, scanning electron microscopy (SEM) and EDS analyses were conducted. SEM examination was conducted to track the reference and conditioned bars microstructural changes. EDS analysis was performed to obtain the change in the bars' chemical composition. To avoid damage to the bar surface, each bar was coated with a layer of epoxy before SEM examination. After the epoxy hardened, the bar was cut into small discs. One face of each disc sample was polished with the help of silicon carbide (SIC) papers with grit number ranging between 180 and 10,000. The SEM and EDS examinations were conducted using a TESCAN MIRA LMS scanning electron microscope by TESCAN in Brno, Czech Republic.
Fourier transform infrared (FTIR) spectroscopy analysis was used to reveal functional group changes in FRP bars components [2]. Powder and cylindrical samples were used for FTIR analysis using the Thermo Scientific Nicolet iS20 spectrometer by Thermo Scientific in Waltham, MA, USA. Powder samples were obtained through sawing the bars and collecting the dust. The saw dust was ground to a very fine powder before the test. The powder was mixed with potassium bromide by grinding them together using a mortar and pestle. Finally, the blended powder was shaped into tablets for FTIR examination. In addition, cylinder samples were prepared by sawing the bars, and the test surface of each specimen was polished using silicon carbide (SIC) papers. All FTIR measurements were conducted using wave numbers from 4000 to 400 cm −1 , and 64 scans were performed with spectral resolution of 4 cm −1 .

Bar Surface Morphology
Typical images of reference and conditioned GFRP and CFRP bars after 180 days of exposure are shown in Figure 4a,b, respectively. One can observe that, after 180 days of immersion, the GFRP bars conditioned at 60 • C in both solutions exhibit noticeable change in color from pale green to dull yellow, but those conditioned at 30 and 45 • C show practically no change. The CFRP bars, irrespective of the solution type or temperature, do not exhibit any obvious color change. The color change per se cannot indicate significant chang and chemical properties as it may be restricted to the resin on t may also depend on the type of pigment used to provide color 3.1.2. Tensile Failure Mode Figure 5a,b, respectively, shows typical failed GFRP and C test. The label on each bar indicates its relevant exposure con Figure 5, the bars exhibited different failure modes. The refere ber type, experienced fiber rupture or interlaminar shear failur conditioned CFRP bars failed similarly. This global failure sp The color change per se cannot indicate significant change in the bar's mechanical and chemical properties as it may be restricted to the resin on the bar surface. The change may also depend on the type of pigment used to provide color to the bar. Figure 5a,b, respectively, shows typical failed GFRP and CFRP bars after the tensile test. The label on each bar indicates its relevant exposure conditions. With reference to Figure 5, the bars exhibited different failure modes. The reference bars, irrespective of fiber type, experienced fiber rupture or interlaminar shear failure outside the anchors. The conditioned CFRP bars failed similarly. This global failure spanned the entire 200 mm conditioned length. The conditioned GFRP bars, however, had a different type of failure. As Figure 5a shows, with the increase in the immersion temperature, the ruptured section became shorter and localized. The local failure could be indicative of the effect of high temperature [2] on the rapid degradation of glass fibers exposed to the solution. Due to defective sizing, flawed fiber-matrix interface, or nonuniform distribution of voids in the matrix, the fibers in the vicinity of the defects would be more readily accessible to the solution. Since glass fibers are susceptible to attack by alkaline/saline solution [63,64], they would suffer more degradation and early rupture under tension. On the contrary, carbon fibers are immune to attack by the chemicals present in the conditioning solutions in the current study.

Retained Tensile Strength and Elastic Modulus of the Conditioned Bars
Three hundred GFRP and CFRP bars were tested under tension. Their strength and elastic modulus values were determined as specified in D7205/D7205M-21 [51] and are reported in Tables 3 and 4, respectively. In each c reported mean strength value and the associated coefficient of variation (COV) ar on data from at least five replicate specimens. In a few cases, the statistical Q-test [ used to reject an outlier.
Before discussing the data in the last tables, Figure 6a,b show the %retained strength of the bars after immersion in solution PS and SS, respectively.

Retained Tensile Strength and Elastic Modulus of the Conditioned Bars
Three hundred GFRP and CFRP bars were tested under tension. Their tensile strength and elastic modulus values were determined as specified in ASTM D7205/D7205M-21 [51] and are reported in Tables 3 and 4, respectively. In each case, the reported mean strength value and the associated coefficient of variation (COV) are based on data from at least five replicate specimens. In a few cases, the statistical Q-test [65] was used to reject an outlier. Before discussing the data in the last tables, Figure 6a,b show the %retained tensile strength of the bars after immersion in solution PS and SS, respectively. As can be noticed, irrespective of the solution type, if the exposure temperature below 45 °C, neither type of bar exhibits more than 5% reduction in tensile strength af up to 90 days of immersion. Between 90 and 180 days, the rate of deterioration increas and the SS solution inflicts greater damage on both types of bar than the PS solution do However, the GFRP consistently suffers higher damage than the CFRP. When exposure temperature is increased to 60 °C, the GFRP experienced dramatic reduction strength, resulting in %retained tensile strength of only 59.3% and 43.1% after 180 days immersion in the PS and SS, respectively. The companion CFRP bar retain approximately 90% of its tensile strength under the same conditions. Although SS inflic slightly higher damage on CFRP than PS did, the difference is relatively small in context of the current test conditions. On the contrary, the damage caused to the GFRP the 60 °C SS was appreciably higher than that caused by the companion PS solution. Sin the two types of bars are made by the same manufacturer using the same type of epo matrix, it can be argued that the glass fiber is susceptible to major damage in CSA concr under high temperature (≥60 °C) and prolonged exposure scenarios.
As carbon fiber is immune from attack by many chemicals, the observed damage the CFRP bar can be attributed to the degradation of the epoxy matrix and/or the fib matrix interface. Based on the relevant ASTM D7957/7957M-22 standard [52], the tes CFRP bar satisfies the alkali resistance requirement of the standard in both the PS and solution, whereas the GFRP bar satisfies the same requirement in the PS solution but n in the SS solution.
In design, another important property of any reinforcement is its elastic modulus severe reduction in the FRP bar elastic modulus would increase the deflection and cra width of FRP-reinforced concrete structures under applied loads. In the current stu none of the exposure conditions had a practically significant effect on the elastic modu of the GFRP bar, but the CFRP bar elastic modulus exhibited approximately 8% reduct after 180 days of immersion. Considering the differences between the GFRP and CF bars' fibers diameters, elastic moduli and volumetric fiber ratios, it can be estimated th under equal tensile load, the interfacial shear stress in the current CFRP bar would be least 50% higher than that in the companion GFRP bar. Consequently, the likelihood interfacial damage at the same tensile force level in the CFRP would be higher than t As can be noticed, irrespective of the solution type, if the exposure temperature is below 45 • C, neither type of bar exhibits more than 5% reduction in tensile strength after up to 90 days of immersion. Between 90 and 180 days, the rate of deterioration increases, and the SS solution inflicts greater damage on both types of bar than the PS solution does. However, the GFRP consistently suffers higher damage than the CFRP. When the exposure temperature is increased to 60 • C, the GFRP experienced dramatic reduction in strength, resulting in %retained tensile strength of only 59.3% and 43.1% after 180 days of immersion in the PS and SS, respectively. The companion CFRP bar retained approximately 90% of its tensile strength under the same conditions. Although SS inflicted slightly higher damage on CFRP than PS did, the difference is relatively small in the context of the current test conditions. On the contrary, the damage caused to the GFRP by the 60 • C SS was appreciably higher than that caused by the companion PS solution. Since the two types of bars are made by the same manufacturer using the same type of epoxy matrix, it can be argued that the glass fiber is susceptible to major damage in CSA concrete under high temperature (≥60 • C) and prolonged exposure scenarios.
As carbon fiber is immune from attack by many chemicals, the observed damage to the CFRP bar can be attributed to the degradation of the epoxy matrix and/or the fiber-matrix interface. Based on the relevant ASTM D7957/7957M-22 standard [52], the tested CFRP bar satisfies the alkali resistance requirement of the standard in both the PS and SS solution, whereas the GFRP bar satisfies the same requirement in the PS solution but not in the SS solution.
In design, another important property of any reinforcement is its elastic modulus. A severe reduction in the FRP bar elastic modulus would increase the deflection and crack width of FRP-reinforced concrete structures under applied loads. In the current study, none of the exposure conditions had a practically significant effect on the elastic modulus of the GFRP bar, but the CFRP bar elastic modulus exhibited approximately 8% reduction after 180 days of immersion. Considering the differences between the GFRP and CFRP bars' fibers diameters, elastic moduli and volumetric fiber ratios, it can be estimated that, under equal tensile load, the interfacial shear stress in the current CFRP bar would be at least 50% higher than that in the companion GFRP bar. Consequently, the likelihood of interfacial damage at the same tensile force level in the CFRP would be higher than that in GFRP. This may explain the higher reduction in the CFRP bar elastic modulus.

Statistical Analysis of Tensile Strength
The following analyses were performed using appropriate statistical procedures [66]. The tensile strength data were analyzed to compare the significance of the differences at the 95% confidence interval. The Shapiro-Wilk test [67], generally used to check normality of a sample size of less than 50 [68], was applied to check the normality of the tensile strength data, and the Levene's test, a standard test for homogeneity of variance [69], was conducted to measure the homogeneity or equality of variances. The appropriate p-value was selected when the null hypothesis, or the alternative hypothesis, regarding the equality of variances was tested. The one-way analysis of variance (ANOVA) was used to determine whether there were any significant differences among at least three levels. The least significant difference (LSD) method was used to conduct post-mortem comparison. For example, the influence of immersion time or exposure temperature was mainly used in one-way ANOVA methods.
All samples used for one-way ANOVA satisfied the normality and homogeneity of variance tests. The Independent sample t-test was used to determine whether there was any significant difference between two levels. When the data did not satisfy the equality of variances, or when they only contained two levels, an independent sample t-test was performed. For both the independent sample t-test and one-way ANOVA, a significance level α = 0.05 was selected. Consequently, in this analysis, any p-value < 0.05 is considered to reflect significant influence.
Based on the Shapiro-Wilk test results, all the GFRP and CFRP samples satisfied the normality condition, except for GPST60D90 and GSST45D90, so the latter two samples were not used in the one-way ANOVA and the independent sample t-test. For assessing the influence of SS versus PS, partial results of the analysis are shown in Table 5. Only the samples with p-value < 0.05, that is, those exhibiting the significant influence of the solution type on their retained tensile strength are listed. For constant solution temperature of 30 • C, the results indicate that the addition of sea salt to the pure CSAC pore solution has no significant effect on the retained tensile strength of either type of bar. As far as the CFRP bar is concerned, solution type has no significant effect on its retained tensile strength, regardless of the length of immersion time. The influence of temperature on retained tensile strength is shown in Table 6. It can be noticed that, for immersion times of 60 days or longer, the GFRP bars immersed in the 60 • C solutions generally exhibit significant differences from those immersed in the companion 30 • C and 45 • C solutions. On the contrary, for immersion periods of less than 90 days, no significant difference is observed between the samples immersed in the 30 • C and 45 • C solutions. As for the CFRP bar, temperature has no significant effect on its retained tensile strength, regardless of the immersion time length.
The effect of immersion time on retained tensile strength is shown in Table 7. With reference to the last table, for either solution maintained at 30 • C, the length of the immersion time has no significant effect on the GFRP bar retained strength. For GPST45, GPST60 and GSST45, only 180 days of immersion has a significant effect. For GSST60, the interval between any of the two consecutive immersion durations shows significant effect. In the case of the CFRP bar, 180 days of immersion shows significant difference with the other immersion times. However, CSST60 exhibits significant difference between 30 and 60 days and between 60 and 90 days. Note: a Y = yes significant; N = not significant at 95% confidence level; b t-test: independent sample t-test was used because data not satisfied homogeneity of variance or data only contained only 2 levels. Note: a Y = yes significant; N = not significant at 95% confidence level; b t-test: independent sample t-test was used because data not satisfied homogeneity of variance or data only contained only 2 levels.
Hence, for the current bars, immersion time, especially 180 days, has a significant effect on the retained tensile strength, while temperature and solution type have significantly more effect on the GFRP bars than the CFRP bars. Figure 7 shows the failure morphology of the GFRP and CFRP bars after the horizontal shear test. As the applied load was continuously increased, longitudinal cracks suddenly appeared, and the load started to decline. Crack formation was accompanied by a loud sound and the release of some epoxy powder. The cracks formed one or more delaminated planes near and parallel to the neutral plane of the bar cross-section. With further increase of the applied vertical displacement, beyond that corresponding to the peak load, more horizontal failure planes formed as illustrated in Figure 7. Eventually, the part of the bar below the lowest plane fractured in the vicinity of the midspan of the specimen. In most specimens, the failure planes formed asymmetrically on only one side of the externally applied load. This horizontal shear failure morphology is similar to that reported by other researchers [26,32,38,41,44,70].

Influence of Span-to-Diameter Ratio on Horizontal Shear Failure
For determining the horizontal shear strength of FRP bars, ASTM D4475-21 [49] suggests testing bar samples with span/diameter ratio not less than 3 nor greater than 6. For a bar of circular cross-section subjected to a pure shear force F, the horizontal shear stress τ acting on any plane located at distance h from the neutral axis can be calculated using Equation (3) [71].
Notice that the maximum shear stress occurs at h = 0. As this standard does not give guidance regarding the selection of a specific ratio, to assess the sensitivity of the current bars horizontal shear strength to this parameter, unconditioned specimens with span-to-diameter ratios of 3, 4, 5 and 6, were tested. The results are shown in Figure 8.

Influence of Span-to-Diameter Ratio on Horizontal Shear Failure
For determining the horizontal shear strength of FRP bars, ASTM D4475-21 [49] suggests testing bar samples with span/diameter ratio not less than 3 nor greater than 6. For a bar of circular cross-section subjected to a pure shear force F, the horizontal shear stress τ acting on any plane located at distance h from the neutral axis can be calculated using Equation (3) [71].
Notice that the maximum shear stress occurs at h = 0. As this standard does not give guidance regarding the selection of a specific ratio, to assess the sensitivity of the current bars horizontal shear strength to this parameter, unconditioned specimens with span-to-diameter ratios of 3, 4, 5 and 6, were tested. The results are shown in Figure 8.

Influence of Span-to-Diameter Ratio on Horizontal Shear Failure
For determining the horizontal shear strength of FRP bars, ASTM D4475-21 [49] su gests testing bar samples with span/diameter ratio not less than 3 nor greater than 6. F a bar of circular cross-section subjected to a pure shear force F, the horizontal shear stre τ acting on any plane located at distance h from the neutral axis can be calculated usin Equation (3) [71].
Notice that the maximum shear stress occurs at h = 0. As this standard does not give guidance regarding the selection of a specific ratio, assess the sensitivity of the current bars horizontal shear strength to this parameter, u conditioned specimens with span-to-diameter ratios of 3, 4, 5 and 6, were tested. The r sults are shown in Figure 8.  As Figure 8 shows, within the above s/d range, the horizontal shear strength decreases almost linearly with the increase in the span-to-diameter ratio. In past works, a similar trend was reported for FRP bars [5,70] and strips [72].
Although horizontal shear test on FRP bars has been conducted by several investigators, only a few have analyzed the influence of s/d. ASTM D4475-21 [49] states that experiments indicate that the horizontal shear strength is a function of the specimen spanto-diameter ratio in most materials. In [70], this parameter was dealt with in detail, and a correction factor was introduced to account for it. According to [70], by knowing the horizontal shear strength of a given bar, τ H , for any for any s/d (3 ≤ s/d ≤ 6), and using it as the reference strength, the corresponding strength other s/d value can be computed as where τ H,S is the predicted horizontal shear strength; c is the correction factor; τ H,R is the reference horizontal shear strength; and S and S R are the target and reference specimen span, respectively. For the present test specimens, using the measured horizontal shear strength for s/d = 6 as the reference, the predicted horizontal shear strength for s/d values of 3, 4 and 5 are computed and plotted in Figure 8. The figure also shows the linear fit to the experimental data and the corresponding R 2 values. It appears that, for s/d values between 3 and 6, the horizontal shear strength varies almost linearly, but Equation (4) has the advantage that it may be applicable to even larger s/d values. Also, it requires horizontal shear strength results for a single s/d value to be able to predict the corresponding strength for any other s/d within the above range.

Horizontal Shear Retention
As mentioned in Section 2.4, the conditioned GFRP and CFRP bars were cut into short test pieces for the purpose of finding the bars' horizontal shear strength. For each exposure condition, six replicates with s/d = 3 were tested. Figure 9a,b shows the retained horizontal shear strength of the conditioned GFRP and CFRP bars, respectively.
Based on Figure 9a, under exposure temperatures of 45 and 60 • C, the SS reduced the retained horizontal shear strength of GFRP more than the PS. On the other hand, exposure up to 60 days generally increased the retained shear strength. When it decreased, the reduction was less than 5%. The largest reduction (≈50%) was experienced in the SS solution after 180 days of immersion at 60 • C. The reduction caused by the PS solution under the same conditions was around 35%. The latter levels of damage are both drastic and are in the same ballpark as the tensile strength reduction experienced by this bar under the same exposure conditions. As both the tensile stress and the horizontal shear stress transfer takes place through the fiber-matrix interface, the high level of degradation in the two strengths can be ascribed to the appreciable degradation of the fiber-matrix interface. On the other hand, the tensile strength is also highly dependent on the fiber strength while the horizontal shear strength is not [35,73]. Consequently, the tensile strength degradation is the consequence of the damage incurred by the fibers and their interfaces.
For the CFRP bar, Figure 9b shows the horizontal shear strength degradation for up to 90 days of immersion in the two solutions. The results beyond 90 days are not available because there was an insufficient number of this type of bar to test. Still, the provided data is useful for comparing the extent of damage to the CFRP relative to the GFRP under the same conditions and for the same immersion duration. Figure 9b shows the retained horizontal shear strength fluctuating with increasing exposure time. This type of fluctuation has been also reported in [34]. It is most likely due to the random variations in the mechanical and microstructural properties of the bar along its length rather than some intrinsic material property. Some of the results seem counter intuitive. For example, the samples immersed in the 30 • C solution exhibit more degradation than the ones immersed in the 60 • C solution. As both the GFRP and CFRP bars exhibit similar fluctuations, it confirms that the fluctuation is not caused by intrinsic material property. Based on Figure 9a, under exposure temperatures of 45 and 60 °C, the SS reduced t retained horizontal shear strength of GFRP more than the PS. On the other hand, exposu up to 60 days generally increased the retained shear strength. When it decreased, the duction was less than 5%. The largest reduction (≈50%) was experienced in the SS soluti after 180 days of immersion at 60 °C. The reduction caused by the PS solution under t same conditions was around 35%. The latter levels of damage are both drastic and are the same ballpark as the tensile strength reduction experienced by this bar under the sam exposure conditions. As both the tensile stress and the horizontal shear stress trans takes place through the fiber-matrix interface, the high level of degradation in the tw strengths can be ascribed to the appreciable degradation of the fiber-matrix interface. O the other hand, the tensile strength is also highly dependent on the fiber strength wh the horizontal shear strength is not [35,73]. Consequently, the tensile strength degradati It should be pointed out that, whereas the tensile test shows the strength of the weakest section along the relatively long free length of the bar, a horizontal shear test reflects the shear strength of the weakest plane of a short segment of the bar or of the plane subjected to combined maximum moment and shear. Unlike the tensile strength, which is not a function of the location of the applied tensile load relative to the position of the weakest plane along the bar, the horizontal shear strength is sensitive to the difference between the moment acting on the weakest plane and the maximum moment acting on the test specimen. Increase in the maximum moment to shear ratio may cause failure at a section other than the weakest section along the shear span.

Influence of Specimen Length
Based on ASTM D7617/D7617M-11 [50], for testing the transverse shear strength of a FRP bar, the length of the test specimen shall be 225 mm even though, theoretically, this type of failure is independent of the specimen length. To investigate the influence of specimen length on the transverse shear strength of the bars in this study, 225, 150 and 100 mm long specimens were tested. At least 8 replicates specimens were tested in each case. The results were statistically analyzed to gauge the influence of the specimen length.
The one-way analysis of variance (ANOVA) was applied to determine whether there was any significant difference among the 225, 150 and 100 mm levels. As Table 8 shows, all p-values of Shapiro-Wilk and Levene's test are larger than 0.05, which signifies that all the data satisfied the normality test and the equality of variances. Also, the p-value of one-way ANOVA for each bar is larger than 0.05, which signifies, as theoretically expected, that the specimen length has no significant influence on its transverse shear strength.

Transverse Shear Strength Retention
Guided by the preceding statistical analysis, the length of the conditioned GFRP and CFRP specimens for the transverse shear test was selected as 100 mm, and five replicate specimens were tested in each case. Figure 10 shows typical CFRP and GFRP samples after transverse shear failure. Regardless of the conditioning environment or the exposure duration, the failure pattern for all the specimens was identical. It was characterized by fractured planes perpendicular to the longitudinal axis of bar.    The transverse shear strength shows similar fluctuation with increasing exposure time as the horizontal shear strength. This phenomenon can be again ascribed to the size effect and the random variation in bar properties along its length. Therefore, the shear strength may fluctuate depending on whether the test section is weaker or stronger than the other sections located outside the test region. The strength of the different sections along the bar can vary due to differences in the cure ratio, the void content, and the nonuniformity of fiber distribution within the bar cross-section.   The transverse shear strength shows similar fluctuation with increasing exposure time as the horizontal shear strength. This phenomenon can be again ascribed to the size effect and the random variation in bar properties along its length. Therefore, the shear strength may fluctuate depending on whether the test section is weaker or stronger than the other sections located outside the test region. The strength of the different sections

Comparison of Mechanical Strengths Degradation
Existing experimental data [24][25][26][27][28][29]31,32,34] have shown that exposure of all types of FRP bars to the same environment generally reduces their tensile and flexural strengths more than their horizontal and transverse shear strengths. These data were mainly collected from tests involving immersion of bars in OPC concrete, or in simulated OPC pore solution, with and without sea salt. The present test results confirm these findings in the case of bars immersed in simulated CSAC pore solution, with or without sea salt.
The horizontal shear test is not commonly used to evaluate the rate of bar degradation [70]. Although horizontal or interlaminar shear strength undergoes reduction under certain conditions, especially under prolonged high temperature exposure scenarios [4,5,[27][28][29]33,38,39,41,74], it also exhibits a high degree of variability for the reasons explained earlier. One study reported a 4% increase in the horizontal shear strength of a GFRP bar after 15 years of embedment in concrete and exposure to real service conditions [75].
Consequently, ASTM D7705/D7705M-12 [48] does not require determination of the effect of FRP bar exposure to aggressive environments on its horizontal or transverse shear strength. It is argued here that the horizontal shear test is not necessary because it does not simulate any of the likely states of stress to which a bonded FRP reinforcing bar may be subjected in a real concrete structure. If shear lag effect in the bar is neglected, a straight reinforcing bar cross-section will be subjected to uniform normal stress only in the horizontal shear test it is put under a bending condition, where normal stress along the height of the section varies linearly, and the middle plane of the bar is subjected to maximum shear. It is difficult to envisage the latter stress state in a reinforcing bar in a reinforced concrete structure. In a bonded reinforcing bar, the change in the normal stress along the bar is equilibrated by the bond stress acting on the bar surface, so from the equilibrium point of view, there is no need for interlaminar shear stress. In an end-anchored unbonded or debonded state, the bar will be subjected to axial tension as in a conventional tension test. Neither of the latter two situations are simulated by the current horizontal shear test.
Past research [27][28][29]33,36] has reported increased reduction of transverse shear strength with the increase in the length of immersion time. In [31], the transverse shear retention of type-C GFRP bars was reported significantly larger than 100%, whereas its tensile strength retention was around 80%. So, it is difficult to obtain a representative retained shear strength using the currently recommended test methods. The current test methods are predicated on the assumption of uniformity of the bar properties along its length; this assumption may not be always satisfied.
As explained in [40], as a result of variations in the bar material properties and manufacturing process, each virgin bar possesses unique microstructures, including distinct voids, defects, and fiber distribution. Whereas these variations may not affect the bar tensile strength because it is determined by the strength of its weakest section along its length, the same is not true in the case of shear strengths. It is therefore suggested that the durability assessment be based on retained tensile strength only.

Influence of Specimen Type
Preliminary FTIR analysis of the matrix powder and mini-cylinders was conducted to select the suitable sample form for further analysis. The sample preparation procedures were detailed in Section 2.5. The raw results of the above analysis for the GR and CR samples are shown in Figure 12. A comparison of Figure 12a,c reveals that the absorbance signal of the powder for both GR and CR is significantly stronger than that of the corresponding cylinder. Although the signals of both samples exhibit clear peaks, the peaks are not the same. Since the cylinder surface could become contaminated during preparation, it was decided to use powder samples for further examination. A comparison of Figure 12a,c reveals that the absorbance signal of the powder for both GR and CR is significantly stronger than that of the corresponding cylinder. Although the signals of both samples exhibit clear peaks, the peaks are not the same. Since the cylinder surface could become contaminated during preparation, it was decided to use powder samples for further examination.
3.6.2. FTIR Results for Powder Specimens Figure 13 and Table 9 show the FTIR spectra and band assignments, respectively.   As Figure 13 shows, the peak of the reference CFRP bar is significantly higher than that of the reference GFRP bar. For example, the peaks for the CFRP bar at wave numbers 1294 cm −1 , 1236 cm −1 and 829 cm −1 are higher than those of GFRP at the same wave numbers. The two types of bars have similar patterns at wave numbers of 4000 cm −1~1 420 cm −1 , but at wave numbers 1420 cm −1~4 00 cm −1 , they do not. The difference is attributed to the interference of the glass fiber in the GFRP bar. Therefore, compared to the FTIR results of the GFRP bar, the results of the CFRP bar are believed to better reveal the characteristic of the epoxy. Table 9. Assignments of the main characteristic absorption bands. Examined   GR  CR  GPS  T60D90   GSS  T60D90   CPS  T60D90   CSS  T60D90 O-H stretching [76,77]  To examine the post-exposure degradation of the two types of bars, the C=C bond of phenyl ring was used as the reference due to its stable chemical characteristic. Using the value of the highest peak as the representative value, the heights of O-H, C-H and C=O relative to that of C=C are shown in Table 10. With reference to the last table, after exposure to the PS or SS solution at 60 • C for 90 days, compared to the reference GFRP bar, the relative contents of O-H, C-H and C=O in the conditioned bar exhibit obvious decrease. In the case of the conditioned CFRP, the relative content of O-H significantly increased, while the C-H and C=O in CPST60D90 and CPST60D90 generally decreased.

Assignment of Wave Numbers to Groups in the Bars
As for the decrease of C=O content in the conditioned GFRP and CFRP bars, as Equation (6) reveals, OH − and Cl − ions will break the double bond between the oxygen and carbon atoms in the ester group, which is macroscopically reflected by increased microcracks and fractures [82]. Since water uptake occurs by using both the epoxy matrix and the fiber-matrix interface [83], the O-H stretching band represents the OH peaks contributed by both the cured epoxy and the absorbed water. As Figure 13 shows, the peak of the reference CFRP bar is significantly higher than that of the reference GFRP bar. For example, the peaks for the CFRP bar at wave numbers 1294 cm −1 , 1236 cm −1 and 829 cm −1 are higher than those of GFRP at the same wave numbers. The two types of bars have similar patterns at wave numbers of 4000 cm −1~1 420 cm −1 , but at wave numbers 1420 cm −1~4 00 cm −1 , they do not. The difference is attributed to the interference of the glass fiber in the GFRP bar. Therefore, compared to the FTIR results of the GFRP bar, the results of the CFRP bar are believed to better reveal the characteristic of the epoxy.
To examine the post-exposure degradation of the two types of bars, the C=C bond of phenyl ring was used as the reference due to its stable chemical characteristic. Using the value of the highest peak as the representative value, the heights of O-H, C-H and C=O relative to that of C=C are shown in Table 10. As for the decrease of C=O content in the conditioned GFRP and CFRP bars, as Equation (6) reveals, OH − and Cl − ions will break the double bond between the oxygen and carbon atoms in the ester group, which is macroscopically reflected by increased microcracks and fractures [82]. Since water uptake occurs by using both the epoxy matrix and the fiber-matrix interface [83], the O-H stretching band represents the OH peaks contributed by both the cured epoxy and the absorbed water.
Based on the above results and their analysis, the writers believe that the reduction in the amount of C=O bonds is a better indicator of epoxy degradation than the change in the amount of OH.

Micromorphology and Chemical Analysis
3.7.1. EDS Mapping Results (6) Based on the above results and their analysis, the writers believe that the reduction in the amount of C=O bonds is a better indicator of epoxy degradation than the change in the amount of OH.

EDS Mapping Results
A rectangular zone approximately 280 µm by 210 µm was used for EDS mapping throughout the current SEM scanning. Figure 14 shows typical SEM images of a conditioned GFRP bar. It shows that elements Si and Al exist mainly in the zones containing glass fiber, while carbon (C) primarily exists in the matrix zone. Oxygen (O) is present in both zones. Na, Cl, K and S seem to be uniformly distributed, but their contents are small.  Detailed EDS results for one cross section of specimen GSST60D90 are shown in Table  11. It should be noted that the element contents in Table 11 are for the whole rectangular zone; they include the contribution of both the fibers and the matrix. By contrast, the results in Figure 14 pertain to a representative area within the rectangular zone. Figure 15 schematically identifies the location and labelling of the zones in the bar cross-section. Detailed EDS results for one cross section of specimen GSST60D90 are shown in Table 11. It should be noted that the element contents in Table 11 are for the whole rectangular zone; they include the contribution of both the fibers and the matrix. By contrast, the results in Figure 14 pertain to a representative area within the rectangular zone. Figure 15 schematically identifies the location and labelling of the zones in the bar cross-section. Table 11 indicates the contents of Na, Cl and K ions in the four zones near the bar surface to be large, while in the zone located at the center of the cross section, they are relatively small. When the penetration depth increases, the quantity of these ions generally diminishes. Accordingly, it is reasonable to state that diffusion of Na, Cl and K ions from the solution is responsible for the higher content of these species in the zones near the GFRP bar surface. Element S, which originates from the SO 4 2− , has relatively high content only in zones Top-1 and Top-2. The largest depth of diffusion in zones Top, Right, Bottom and Left are about 630, 1260, 840 and 630 µm, respectively, which means that the diffusion depth is not uniform. This nonuniformity is believed to be due to the random variations in the bar cross-section microstructure and properties, a phenomenon that needs to be considered in FRP durability models that normally assume a constant diffusion depth in a bar.  Detailed EDS results for one cross section of specimen GSST60D90 11. It should be noted that the element contents in Table 11 are for the zone; they include the contribution of both the fibers and the matrix. sults in Figure 14 pertain to a representative area within the rectangu schematically identifies the location and labelling of the zones in the b   The EDS mapping results for one cross section of specimen CSST60D180 are shown in Table 12. The Na, Cl, and K ions contents in the Top-1 and Right-1 zones are significantly larger than those at the center of the cross section. Going along the radius from Top-1 towards the center, the preceding ions contents fluctuate albeit they generally decrease. On the other hand, the S element content is negligible. The largest depths of diffusion in the Top, Right, Bottom and Left directions are about 1680, 210, 210 and 210 µm, respectively, which highlights the nonuniformity of the diffusion depth. The SEM image of an entire cross-section is shown in Figure 16a, where four distinct bandings can be observed. The clearest banding, which is believed to show the diffusion path, coincides with the zone in the top part of the bar where fluctuations in the Na, Cl and K ions contents were detected. To confirm the above assertion, additional points within the cross-section, as shown in Figure 16b, were examined, and the results are shown in Table 13. The results indicate that the Na, Cl and K ions contents in the darker regions are significantly larger than the corresponding contents in the lighter regions. This can be adduced as further evidence in support of the darker regions being diffusion paths. The SEM image of an entire cross-section is shown in Figure 16a, where four distinct bandings can be observed. The clearest banding, which is believed to show the diffusion path, coincides with the zone in the top part of the bar where fluctuations in the Na, Cl and K ions contents were detected. To confirm the above assertion, additional points within the cross-section, as shown in Figure 16b, were examined, and the results are shown in Table 13. The results indicate that the Na, Cl and K ions contents in the darker regions are significantly larger than the corresponding contents in the lighter regions. This can be adduced as further evidence in support of the darker regions being diffusion paths.    Diffusion of Na, Cl and K ions in the CFRP bar seems unusual. As Figure 16a shows, the diffusion does not occur radially as normally assumed; rather, it seems to occur along a series of horizontal secant lines. This phenomenon has not been previously reported and needs more investigation. If diffusion were to occur in this manner, then current diffusion models, which assume diffusion radially, would not be able to correctly predict the depth of the ion's penetration.
EDS mapping results for one cross section of CSST60D90 are shown in Table 14. The locations of Z1, Z2 and Z3 are shown in Figure 16c. As this table shows, the content of the Na, Cl and K ions decreases from Z1 to Z2 and from Z2 to Z3. Once again it points to the fact that diffusion occurred along the Z1-Z2-Z3 path, rather than radially. Representative micro-morphologies of the reference and conditioned GFRP and CFRP bars are shown in Figure 17. Figure 17a,b reveals the presence of microcracks in GR. Figure 17e indicates that, after conditioning, crack density in GSST60D30 increased as new cracks formed and joined preexisting cracks. After 90 days of conditioning, severe defects, such as pits or tiny cavities (Figure 17f), formed, and the interface between the glass fibers and the epoxy matrix degraded, which is highlighted by the white lines encircling the glass fibers in Figure 17f. Figure 17i shows that the epoxy around the glass fibers in the degraded region almost vanished after 180 days of exposure. Figure 17c,d indicates that the interface between the carbon fibers are sound, but certain defects exist within the matrix. After 90 days of exposure to the PS solution at 60 • C, as Figure 17g shows, the number of defects at the surface of the bar increased significantly. Figure 17h shows that the epoxy in the defective zones almost totally dissolved. Figure 17j,l shows some deposits on the surface of carbon fibers, while Figure 17k shows that, after 180 days of exposure, the defects propagated toward the inner region of the CFRP bar.  Figure 18 shows representative locations of the points examined by EDS in the conditioned GFRP and CFRP bars. The relevant element contents are given in Table 15. The data in Table 15 show that the main elements (note these may be in ionic form) in glass fiber are Si, O, Ca and Al, which broadly agrees with the EDS findings. The main element in the carbon fiber is correctly identified as C. The main elements in the GFRP matrix are identified as C, O and Zr, while those in the CFRP matrix are identified as C and O. Only around 2% Zr was found in CSST60D180.   The data in Table 15 show that the main elements (note these may be in ionic form) in glass fiber are Si, O, Ca and Al, which broadly agrees with the EDS findings. The main element in the carbon fiber is correctly identified as C. The main elements in the GFRP matrix are identified as C, O and Zr, while those in the CFRP matrix are identified as C and O. Only around 2% Zr was found in CSST60D180.

Chemical Analyses
As for the glass fiber, the element contents at GF2, located at the fiber center, do not show an obvious change post immersion. At GF1, located near the fiber surface, Si and Ca contents show a decrease. On the other hand, the Si, Ca, Al, Na, K, S and Cl contents in the matrix at points GM1 and GM2 all show an increase to different degrees. The Si and